A device using various types of disks, such as optical disks, magneto-optical disks, or flexible magnetic-recording disks, is known as a disk drive. Of these, a hard-disk drive (HDD) is used in many electronic devices, such as video recording and playback devices and car navigation systems, in addition to computer systems.
A magnetic-recording disk used in a HDD has a plurality of data tracks and a plurality of servo tracks formed in concentric circular shapes. Each servo track is included of a plurality of servo sectors containing address information. In addition, each data track is included of a plurality of data sectors containing user data. The data sectors are recorded between the servo sectors which are separated in the circumferential direction.
A HDD includes a rotary actuator, and a head-slider that is supported by the actuator. The HDD reads out the address information in the servo sector by using the head-slider, and controls the actuator in accordance with the address information, which is servo-control information. Thus, the HDD can move the head-slider, in an operation known by the term of art, “seek,” to the desired radial position, which is a target data track, and position the head-slider at the target data track, in an operation known by the term of art, “track following.” The head-slider positioned at the target data track writes data to, or reads data from, the target data sector in the track.
The actuator is driven by a voice-coil motor (VCM). Generally, the actuator is known to exhibit mechanical resonance at a specific frequency, referred to as a resonant frequency. In the servo control of the head-slider, which is attached to the actuator, when the actuator starts to resonate mechanically at the resonant frequency, the amplitude at the resonant frequency is superimposed on the servo signal which is read out, and the servo control of the head-slider becomes unstable.
Therefore, a conventional HDD uses a notch filter in the servo control of the head-slider, as is known in the art. The notch filter is inserted on the output side of the controller in the servo-control loop of the head-slider, and has a center frequency that is the same as the resonant frequency of the actuator. The notch filter lowers the servo gain at the resonant frequency included in the control signal, and stabilizes the servo control of the head-slider.